1. Field of the Invention
The present invention relates to a method for producing printing forms with irregularly distributed printing points in which an original image is opto-electrically scanned in individual surface elements with the desired detailed resolution, whereby each surface element has a gradation value corresponding to the scanning signal assigned thereto and surface elements having the same or approximately the same gradation value are combined into sub-surfaces and, corresponding to their gradation value, the individual subsurfaces are reproduced by a plurality of statistically-distributed printing points whose overall area coverage within the sub-surface corresponds to the gradation value of that sub-surface.
2. Description of the Prior Art
A method for producing printing forms has already been proposed in the German patent application P No. 29 31 0928 (corresponding to PCT-OS No. W0 80/00110), fully incorporated herein by this reference, in which a statistical or quasistatistical (stochastic) gradation dependent distribution of the smallest printable points is undertaken on the recording page by way of a random generator. This method operates according to the following steps.
Opto-electrical scanning of the master according to reference surfaces into individual surface elements whose size is determined by the detailed resolution desired in the reproduction, for example 100.times.100 .mu..sup.2, whereby, for example, 6.times.6 surface elements form a reference surface which corresponds to a recaording area in which, for example, 12.times.12 of the smallest still printable points of the size 50.times.50 .mu..sup.2 are reproducible.
Comparison of the density values within each reference surface obtained during the scanning.
Combination of surface elements of nearly identical density into surface passages having respective, common, mean density.
Identification of the plurality of smallest printable points within each surface passage corresponding to the respective density value, whereby the positions of the points in a printing matrix which serves for the control of the recording are determined by a random process.
Recording in accordance with the data stored in the matrix in a known manner by recording devices standard in scanner technology, for example an exposure unit according to the German Letters Pat. No. 2,107,738, corresponding to U.S. Letters Pat. 3,725,574, fully incorporated herein by this reference.
On the scanning page, a reference surface which is scanned in smaller surface elements corresponds to the size of the printing matrix in this method.
It has been proven, in practice, that surface elements of approximately the same gradation value occasionally extend beyond the reference surface and, therefore, beyond the size of the printing matrix as well, whereby such a surface is sub-divided during recording into equal-sized rectangles or squares. As a result, either a higher-order raster of the size of the reference surface or, respectively, printing matrix arises, which can potentially lead to periodic repetitions of the disposition of the printing point, or the printing matrix must be selected of such size that a periodic repetition within a master is not to be anticipated.